The present invention relates to surfacing materials and more particularly to powder-cored strip electrodes for use in surfacing with abrasion-resistant composite alloys.
The invention is of particular advantage for hardening the face of parts subject to excessive wear and operating at elevated temperatures under the effect of aggressive medium, or exposed to abrasive or gas-abrasive wear. Thus, the elements of blast furnace charging arrangements: valves, bells, cups, etc, manufactured from various steel grades are subjected in service to such severe wear.
Intensification of the blast furnace process (an increase in gas pressure in blast-furnace tops up to 3 atm.g.p., a rise in air blast temperatures of up to 1000.degree. C, the use of pellets and sinter with high abrasion characteristics as raw materials) causes a sharp increase in the wear of the elements of the charging devices.
To extend the service life of the above-mentioned parts they are given additional strength by applying abrasion-resistant alloys by the arc process. This surfacing practice has found wide use now.
An example of such abrasion-resistant alloys are cobalt-chromium-tungsten-base and nickel-molybdenum-chromium-base doped alloys which came into use in industry.
Abrasion resistance of the above alloys is effected by the presence in their structure of strengthening solid phases. The latter are multiply-doped tungsten and chromium carbides and intermetallics of said elements. The strengthening phases are formed either in the course of crystallization of a homogeneous melt or during heat treatment of the deposited layer. The amount and nature of the strengthening phase depend on the whole range of interrelated physicochemical phenomena, particularly on the solubility limits of alloy components in the metal base of the surfaces abrasion-resistant alloy. The above-mentioned physico-chemical phenomena are practically uncontrollable in the course of surfacing with strengthening alloys and often cause the formation of complex phases enriched with the base metal of a part being hardened, which to a certain extent diminishes abrasion resistance of the deposited layer.
The above-specified doped alloys are applied to parts to be hardened by the mechanized submerged arc process with powder-cored electrodes or solid rods, with the part being preheated to a temperature ranging from 160.degree. to 320.degree. C and maintained at this temperature for the complete time of surfacing.
A main disadvantage of said alloys is their inadequate abrasion resistance, through which the service life of a blast furnace charging cup and bell surfaced with the above alloys amounts, as a rule, to one year and a half or two years for blast furnaces with higher gas pressures of up to 1.5 atm. Another example of the abrasion-resistant alloys is high-chromium iron of the "Sormite-I" type having the following composition, per cent by weight: carbon, 2.2-3.2; chromium, 22-27; silicon, 2.2-3.2; nickel, 2.2-3.2; manganese, 1.7-2.5; iron the balance. The alloys of the Sormite-I type are applied to parts being hardened with the aid of electrode materials in the form of electrodes with a protective coating, powder-cored electrodes or strip electrodes employed for the automatic arc surfacing process (see, e.g. French Pats. No. 2,142,259, the Author's Certificate of the USSR No. 300281). A main disadvantage of the above-mentioned alloy is its brittleness which inevitably causes the formation of a network of cracks in the applied layer which constitute most probable loci of abrasion. Moreover, peculiar to the alloy of the Sormite-I type is deterioration in hardness at elevated temperatures. Thus, at a rise in temperature from 20.degree. to 550.degree. C the Brinell hardness diminishes from 580 to 350 units.
As a result, charging devices hardened with Sormite-I have an inadequate service life ranging within one or one and a half years. Since the operating period of a blast furnace between overhauls amounts to 5-10 years, the losses due to frequent replacement of charging arrangements within that period comes to hundreds of thousands of dollars per furnace.
Composite alloys are to a greater extent free from the above disadvantages, said composite alloys being a composition which consists of a granular abrasion-resistant strengthening phase and a matrix-alloy, and in which, in contrast to the doped alloys, the amount and nature of the strengthening phase is prescribed depending on the requisite abrasion resistance.
A composite alloy is a combination of at least two chemically heterogeneous materials with a boundary therebetween. The composite alloy is formed due to volumetric combination of the above-specified heterogeneous components.
The composite alloy features properties not inherent in its components taken separately.
Known in the art is the composition of such a composite alloy, comprising the strengthening phase -- the grains of cast tungsten carbides, and matrix-alloy of the following composition (weight per cent): nickel, 19-20; manganese, 18-20; iron, up to 1; copper, the balance (see, e.g. GDR Pat. No. 79408). In the above alloy the grains of the strengthening phase may range from 0.18 to 2.0 mm in cross-section, their volume concentration varying usually within 45-70%.
A combination of high hardness of the cast tungsten carbide grains approximating that of diamonds, with high strength and ductility of the above-mentioned copper-base matrix-alloy ensures high abrasion resistance of the known composite alloy.
The composite alloy of the above-specified composition was applied to the valves, cups and bells of blast furnace charging arrangements (see a magazine "Metallurgist" No. 1, 1973). Their service life was 3 to 4 times as great as that of similar parts surfaced with the alloy of the Sormite-I type.
The elements of blast furnace charging arrangements were surfaced with the above-described composite alloy with the aid of a mould which was welded to the part being surfaced so that a clearance of a preset size was formed therebetween, with the size corresponding to the thickness of the applied layer. The clearance was filled with granular tungsten carbide rammed down by a tamper. The matrix-alloy in the form of metal castings was placed in a special hopper above the strengthening phase. Next the surfacing zone was sealed by welding the mould and part together with a tight weld whereupon they were placed into a furnace where they were heated to a temperature of 1200.degree. C without the access of oxygen from ambient air. At the above temperature the matrix-alloy melted, ran down and wetted the grains of the tungsten carbide and the surfaces of the part and mould. The part being hardened and the mould were held at the above temperature for 1-3 hours and then cooled in the furnace.
When surfacing the bell and the cup the duration of the technological cycle was equal to about 48 hrs. The subsequent aging of the deposited layer was effected at a temperature of 400.degree.-450.degree. C and lasted 20-40 hours.
The mould was removed from the finished part by machining with the help of a cutter, and the part contact zone was ground to required geometrical size.
A comparatively low temperature of the surfacing process allows maintaining in the deposited layer the prescribed concentration of the strengthening phase which retains its physicochemical properties. In this case the base metal of the part is not melted which ensures the preservation of prescribed characteristics of the composite alloy.
However, the use of this method, referred to hereinafter as the furnace surfacing technique, is limited by the process temperature which in surfacing steel parts should not exceed 1200.degree. C. This is a limitation restricting the selection of the composite alloys, since the melting points of their matrix-alloys must not exceed the above-specified temperature of the surfacing process.
Other disadvantages of the above-disclosed method involve a complicated and labour-consuming technique which calls for the complete sealing of the surfacing zone.
Moreover, inherent in the above-outlined furnace surfacing technique is a high power input which is attributable to the need in heating heavy metal masses (about 25-50 t). The specific power consumption per 1 kg of the deposited metal varies from 75 to 150 kWhr/kg. Another disadvantage resides in high metal consumption which can be put down to single-application metal moulds. The need for unique heat-treating equipment can be also considered as one of the disadvantages of the above method.
Mechanized surfacing with the strengthening alloys by the arc process is free from the above disadvantages of the furnace surfacing procedure with the use of additional moulds.
However, owing to the lack of adequate electrode materials surfacing by the arc welding process did not find wide industrial application for hardening parts with composite alloys. Thus, known in the art is a tubular electrode for surfacing with a composite alloy, said electrode comprising a protective coating which is a steel casing shaped as a tube, and a powdered mixture or charge which contains tungsten carbides, nickel and manganese, the weight percentage of the components being as follows: nickel, 4-5; manganese, 1-1.5; tungsten carbides, the balance (see, e.g., Author's Certificate of the USSR No. 390899).
As regards the use of the above-described electrode with a protective coating for surfacing with composite alloys, it should be noted that it suffers from a number of serious disadvantages.
A main disadvantage of the known electrode lies in that surfacing is associated with a partial or complete dilution of the strengthening phase -- tungsten carbides -- in molten metal of the electrode steel casing. This is attributable to a high melting point of the iron-base matrix-alloy under the conditions of surfacing by the arc process. It results in a reduction in the preset concentration of the strengthening phase and in a change in the composition of the matrix-alloy caused by the dilution of the tungsten carbides in steel. The structures formed in this case vary from tungsten steel to iron with a considerable amount of secondary tungsten-ferrous carbides. Said structures feature an enhanced brittleness and show a tendency toward cracking the spalling of the strengthening layer built-up with the above-described electrode which diminishes abrasion resistance of parts being hardened.
Moreover, the use of the protective coating for the above electrode precludes mechanization of the surfacing process, and the removal of a slug crust formed on the applied hard face calls for additional technological operation.